In an attempt to block email spoofing attacks on yahoo.com addresses, Yahoo began imposing a stricter email validation policy that unfortunately breaks the usual workflow on legitimate mailing lists.

The problem is a new DMARC (Domain-based Message Authentication, Reporting and Conformance) "reject" policy advertised by Yahoo to third-party email servers, said John Levine, a long-time email infrastructure consultant and president of the Coalition Against Unsolicited Commercial Email (CAUCE), in a message sent to the Internet Engineering Task Force (IETF) mailing list Monday.

DMARC is a technical specification for implementing the SPF (Sender Policy Framework) and DKIM (DomainKeys Identified Mail) email validation and authentication mechanisms. These technologies were designed to prevent email address spoofing commonly used in spam and phishing attacks.

The goal of DMARC is to achieve a uniform implementation of SPF and DKIM among the top email service providers and other companies that want to benefit from email validation.

The specification introduces the concept of aligned identifiers, which requires the SPF or DKIM validation domains to be the same as or sub-domains of the domain for the email address in the "from" field. The domain owners can use a DMARC policy setting called "p=" to tell receiving email servers what should happen if the DMARC check fails. The possible values for this setting can be "none" or "reject."

Over the weekend Yahoo published a DMARC record with "p=reject" essentially telling all receiving email servers to reject emails from yahoo.com addresses that don't originate from its servers, Levine said.

While this is a good thing from an anti-spoofing perspective, it raises problems for legitimate mailing lists, according to the email expert.

"Lists invariably use their own bounce address in their own domain, so the SPF doesn't match," Levine said. "Lists generally modify messages via subject tags, body footers, attachment stripping, and other useful features that break the DKIM signature. So on even the most legitimate list mail like, say, the IETF's, most of the mail fails the DMARC assertions, not due to the lists doing anything 'wrong'."

With the new policy, when a Yahoo user sends an email to a mailing list, the list's server distributes that message to all subscribers, changing the headers and breaking DMARC validation. List subscribers with email accounts on servers that perform DMARC checks, such as Gmail, Hotmail (Outlook.com), Comcast or Yahoo itself, will reject the original message and respond back to the list with automated DMARC error messages.

For example, Gmail will respond with a message that reads: "smtp;550 5.7.1 Unauthenticated email from yahoo.com is not accepted due to domain's DMARC policy. Please contact administrator of yahoo.com domain if this was a legitimate mail."

So users of Gmail, Hotmail and other DMARC-enabled providers will not only fail to receive messages sent to the mailing list by Yahoo users, but will flood the list with bounce messages, risking to be bounced off the list themselves, Levine said.

The email expert recommended that mailing list operators suspend the list posting rights of yahoo.com users and ask them to re-subscribe to their lists with accounts from different email providers.

Yahoo did not immediately respond to an inquiry seeking clarification on whether its new DMARC policy is permanent or temporary.

A test of Yahoo's DMARC records Tuesday done with a tool on dmarcian.com revealed that the "p=reject" setting was still in place for the yahoo.com domain. By comparison, gmail.com had a policy record of "p=none," meaning it doesn't tell other email servers how to handle messages from gmail.com addresses that fail DMARC checks.

Laura Tessmer Atkins, co-founder of email anti-spam consultancy firm Word to the Wise based in Palo Alto, California, also confirmed and documented the issue in a blog post Monday. She believes that Yahoo began advertising a "reject" policy because of a recent attack against Yahoo users that involved attackers compromising yahoo.com email accounts and sending unauthorized emails to their contacts.

"The attackers have modified their attacks and are now sending mail from Yahoo users to their contacts through other servers," Atkins said. "By publishing a p=reject record, Yahoo is telling other systems to not accept mail from Yahoo users if it doesn't come through Yahoo controlled servers. This includes the mail from the attackers, but also mail from regular Yahoo users that use another SMTP server, including bulk mail sent through ESPs [email service providers], and individual mail sent to mailing lists."

DMARC.org, the industry group that oversees the development and adoption of the DMARC standard, did not immediately respond to a request for comment about the Yahoo situation. However, the frequently asked questions section of the group's website acknowledges the interoperability problems mailing lists can have with DMARC and offers some recommendations.

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